This section provides background information related to the present disclosure which is not necessarily prior art.
In addition, a so-called front circuit for a capacitive sensor comprising the pulse generator according to the disclosure is provided.
Capacitive sensors are widely spread not only in the automation technology, but more recently also in the automotive industry, where they find application, inter alia, as trunk or door opener or in seat occupancy detection.
They have at least one electrode capacitance which can be influenced from the outside, the electrodes of which are supplied with a high frequency alternating current. The changes in capacitance caused by an object entering into the alternating electric field are usually converted into a pulse of variable length in order to improve the further processing. This kind of devices are also manufactured and sold by the applicant.
In order to avoid faulty operations, a plurality of electrodes are used which allow a more accurate analysis of the situation. For example, rain, snow or falling leaves or unintentional human influences, such as passing people, must be clearly distinguished from a hand or a foot approaching in a defined way.
Such a capacitive sensor arrangement including a charge pulse output is referred to in the literature as front-end electronics.
For its operation it requires a pulse generator for generating short electrical pulses. A suitable pulse generator is shown in DE 103 59 441 A1.
Here, however, the pulse duration is not adjustable, which is disadvantageous in realizing different measuring tasks with different pulse lengths.
In addition, this circuit is not suitable for driving digital components, which is also regarded as a disadvantage.
A circuit for adjusting the pulse width of the clock signal for a microprocessor called “impulse width timing circuit” is shown in US 2011/0 080 201 A1. Here a square-wave input signal is supplied to two complementary RC elements. Herein, complementary means that a capacitance can be recharged quickly via a first resistance path and slowly via a second resistance path acting reversely. In the above-mentioned document, a variable delay time is set by a variable capacitance and a pulse duration by variable capacitances and/or resistances. The tuneable resistances and capacitances are considered as disadvantageous.